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Article

Quantifying the Potential Co-Benefit of Air Quality Improvement on Cultural Heritage in China

by
Ximo Wang
1,
Huimin Li
1,*,
Yufei Wang
2 and
Xiaofan Zhao
3
1
Beijing Climate Change Response Research and Education Center, Beijing University of Civil Engineering and Architecture, Beijing 100044, China
2
Department for Consulting and Research, Management World Journal, Beijing 100026, China
3
Division of Public Policy, The Hong Kong University of Science and Technology, Hong Kong, China
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(11), 8709; https://doi.org/10.3390/su15118709
Submission received: 24 April 2023 / Revised: 18 May 2023 / Accepted: 25 May 2023 / Published: 28 May 2023
(This article belongs to the Section Tourism, Culture, and Heritage)
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Abstract

:
Atmospheric pollutants can corrode heritage materials, especially stone, which can cause a great loss that goes far beyond the economic losses of the degraded materials. Over the past decades, conventional air pollutants have been slashed owing to clean air actions in China, which produces a significant co-benefit for heritage conservation. However, the benefits may be offset by increases in the photochemical oxidants in smog, such as ozone, which damage heritage materials. This study employed dose–response functions to quantify the impacts of air pollutants on the surface recession of the limestone of heritage structures in China, and assessed the potential benefits of air quality improvement for heritage conservation. The results show that the annual recession rate decreased from 9.69 μmy−1 in 2006 to 6.71 μmy−1 in 2020, resulting in a 41.4% increase in the number of heritage sites meeting the ICP Materials (International Co-operative Program on Effects on Materials including Historic and Cultural Monuments) control target of 8 µmy−1 for 2020. The air quality improvement avoided CNY 136.2 million in heritage site maintenance costs. The recession risk shows distinct regional differences; the southern and northwest regions are still at a higher material corrosion level than the northern and Qinghai–Tibet regions. Nationwide, PM10 (particles with aerodynamic diameter less than 10 μm) is the main risk factor responsible for the surface recession of limestone material of heritage structures in China. The study provides evidence for the benefits of air quality improvement for heritage conservation. Further, the study also puts forward policy recommendations for heritage conservation, including assessing pollution risk, promoting heritage conservation through social sustainability, and implementing differentiated conservation strategies.

1. Introduction

Cultural heritage has always received scientific interest due to its aesthetic, historic, scientific, cultural, economic, social, and spiritual significance [1,2]. The proactive conservation of cultural heritage can greatly contribute to the sustainable development of our society [3]. However, the aspirations of preserving heritage are often at odds with many factors that endanger them. Air pollutants, although they may have historical or aesthetic significance in heritage [4,5], are generally considered to be unfavorable environmental factors that put cultural heritage at varying degrees of risk [6]. With the evolution of industry and socioeconomic activities, various irreplaceable aspects of cultural heritage, both in outdoor and indoor environments, have experienced degrading effects of air pollution [7,8], leading to increasing scientific research about the corrosion effects of air pollution on cultural heritage [9,10,11], as well as the remarkable cultural and economic consequences [12,13].
UNESCO has identified air pollutants as one of the main factors threatening cultural objects, especially those exposed to atmosphere (https://whc.unesco.org/en/factors/ (accessed on 1 October 2022)). The effect of air pollution on cultural heritage has been researched for decades in Europe [14,15]. Studies provide information on non-metallic material damage due to air pollution [16,17] and on the corrosive effects of acid precipitation on metals [18]. Some studies and courses focus on stone and summarize published material, providing a framework for building a coherent base of useful knowledge for practicing conservators and scientists [11,19,20]. Air pollutants accelerate the natural degradation of materials and lead to great losses, including maintenance, repair, renovation, and cleaning costs [13,21]. Relevant studies have shown that the air pollutants most relevant to material decay are carbon dioxide (CO2), nitrogen oxides (NOx), sulfur oxides (SO2), particulate matter (PM), and ozone (O3) [22,23]. The pollutants are deposited from the atmosphere in two ways; one is wet deposition (by forming acidic compounds and falling to the Earth as precipitation or snow), and the other is dry deposition (by forming settleable particles, aerosols, and gases, or incorporating into dust or smoke) [6,13,24]. Both depositions can cause significant deterioration of materials. SO2 is considered to be the most relevant pollutant regarding material degradation, and can trigger electrochemical corrosion of metals or react with alkaline substances in stone in the presence of water or moisture, resulting in the formation of compounds such as oxides, hydroxides on metals, and soluble gypsum crusts on stones [25,26,27]. The deleterious effects can be greater in combination with NO2 and O3 as they are highly oxidizing and also corrode heritage materials. NO2 can be converted into gaseous or liquid nitric acid, which reacts chemically with metal and limestone materials [28]. O3 is the major component of photochemical smog and is regarded as a secondary pollutant, which can lead to the degradation of organic materials and the corrosion of various metal materials [29,30]. A growing body of literature has discussed the importance of carbon compounds (elemental carbon and organic carbon) of PM (and their measurement) in the degradation process of heritage materials [25,31,32]. The carbon compounds make buildings dirty by soiling and blackening monumental surfaces [33,34], and can also accelerate corrosion through their involvement in a number of chemical reactions [8]. Carbon compounds as well as nitrate and sulfate compounds were monitored in glass samples [35], confirming the effects of CO2, SO2, and NO2 on glass. Although CO2 is not considered a pollutant, it is the most emitted acidic gas and is considered to be the main factor responsible for the carbonation of concrete and the surface recession of carbonate materials [36].
The degradation of cultural heritage is a complex process related to the physical–chemical properties of the building materials, the meteorological conditions and air quality in the immediate vicinity of the site, etc. [37,38,39]. For example, carbonate stone may be more chemically sensitive to water, acid rain, and gaseous pollutants compared to silicate rocks, which are more chemically stable [40,41]. The synergistic action of air pollutants and unfavorable climatic parameters can significantly exacerbate the natural degradation rate of materials [42], and in heavily contaminated areas, the dry deposition of the pollutants may be relatively more important.
For measuring the relationship between the corrosion rate and the pollutant level, scholars have developed several dose–response functions (DRFs), including the Lipfert function, Livingstone function, Webb function, Baedecker function, etc. [43,44,45,46]. The early DRFs usually focus on SO2 concentration, as it was the primary pollutant and the major corrosion stimulus in earlier years. The International Co-operative Programme on Effects on Materials including Historic and Cultural Monuments, abbreviated as ICP Materials (https://www.ri.se/en/icp-materials (accessed on 1 October 2022)), is a collaborative project supporting the Convention on Long-range Transboundary Air Pollution (CLRTAP) with science for policy. The project currently involves 16 countries, mainly in Europe. Aiming to investigate the effects of air pollution on the corrosion of construction materials and cultural heritage, ICP Materials conducted the MULTI-ASSESS project [47] and developed several equations to reflect the physical–chemical corrosion mechanisms, including the synergistic effects of SO2 and O3 [16,48]. In 1997–2001, a group of new dose–response functions were developed based on data from the ICP Materials. These functions differ from previous functions by considering the development a new multi-pollutant situation in Europe [49].
Based on these DRFs, several researchers have quantified the relationship between pollutants and materials in both Europe and North America, from the city scale [23,30,50,51] to nationwide [36,50,52]. The current research shows that the recession risk of cultural heritage is serious, although the degradation rate of the materials of cultural heritage sites has decreased over the past few decades due to air pollution control. As one of the countries with the most types of heritage sites, the number of immovable cultural relics in China has reached 767,000 [53]. With a vast territory and diverse climate and environment, the cultural heritage sites in China are challenged by the constant threats of atmospheric pollution and greenhouse gases. However, few researchers have focused on the relationship between air pollution and heritage conservation in China. Li [54] examined the effects of pollutants on the deterioration of Pit 1 of the Emperor Qinshihuangs Mausoleum Site Park, a World Heritage Site, by comparing two years of comprehensive monitoring data of pollutants. Xiao et al. [55] estimated pollutant damage of the world cultural heritage sites in China by the thermal inversion method.
Over the past few decades, China has implemented a series of policies to battle air pollution [56,57,58], and pollutants have been reduced greatly nationwide. However, this has been accompanied by an increase in other oxidants, leading to a multi-pollution situation in China with significant spatial and temporal differences [59]. China’s complex atmospheric environment, vast landscape, and rich variety of cultural objects emphasize the need for a deeper understanding of the pollutant damage to heritage sites. This study estimated the recession rates of limestone material of heritage structures due to air pollution in 2006–2020 and quantified the benefit of pollutant reduction to cultural heritage conservation. This study provides evidence for the benefits of air quality improvement for heritage conservation and contributes to proactive heritage conservation strategies.
The rest of this paper is organized as follows. Section 2 presents the methodology for quantifying air pollutants and heritage recession. Section 3 assesses the impact of air pollutants on the surface recession of heritage structures and estimates the benefits of clean air action for heritage conservation. Section 4 discusses the policy implications and illustrates the limitations of the study. Finally, the paper is concluded with the main findings in Section 5.

2. Methodology

2.1. Scope

The Law of the People’s Republic of China on the Protection of Cultural Relics grades heritage sites at the national level, the provincial level, and the city or county level, based on their historical, artistic, and scientific value [60]. This study involved 5058 heritage sites at the national level in China (excluding Hong Kong, Macao, and Taiwan). Their data were extracted from the National Cultural Heritage Administration of China [61], including the name, type, location, etc., of the sites. For highlighting the World Heritage Sites, which are selected by the United Nations Educational, Scientific and Cultural Organization (UNESCO) and usually included in the sites at the national level, the study involved 35 World Heritage Sites on the 2017 UNESCO World Heritage List. Currently, the number of World Heritage Sites in China has reached 56. The World Heritage Sites in China inscribed on the UNESCO World Heritage List were obtained from the website of the World Heritage Convention [62]. The spatial distribution of the national cultural heritage sites and the World Cultural Heritage Sites is shown in Figure 1.

2.2. Methods

This study used the dose–response functions proposed by the MULTI-ASSESS project [47] to quantify the impact of air pollution on the surface recession of heritage structures. The functions express the surface recession or corrosion depth of Portland limestone after one year of exposure [63]. The function is as follows:
R = 3.95 + 0.0059 ∗ [ S O 2 ] ∗ R h 60 + 0.054 ∗ R a i n ∗ [ H + ] + 0.078 ∗ [ H N O 3 ] ∗ R h 60 + 0.0258 ∗ P M 10
Considering that nitric acid (HNO3) is not frequently measured in the regular monitoring networks, the MULTI-ASSESS project proposed a method to estimate the concentration of HNO3 by NO2, O3, relative humidity (Rh), and temperature (T) [47,63], and the equation is as follows:
[ H N O 3 ] = 516 ∗ e − 3400 / ( T + 273 ) ∗ ( [ N O 2 ] ∗ [ O 3 ] ∗ R H ) 0.5
where
  • R = surface recession or corrosion depth, μm;
  • Rh = annual average relative humidity, %;
  • Rh60 = Rh–60 when Rh > 60, and 0 otherwise;
  • Rain = annual amount of precipitation (mm year−1);
  • T = annual average temperature, °C;
  • [SO2] = annual average SO2 concentration, μg m−3;
  • [HNO3] = annual average HNO3 concentration, μg m−3;
  • [O3] = annual average O3 concentration, μg m−3;
  • [PM10] = annual average PM10 concentration, μg m−3;
  • [H+] = annual average H+ concentration in rain, mg L−1.
The damage function was derived from field experimental exposures including substantial testing and observation of stone samples in the EU MULTI-ASSESS project [47], which can adequately reveal the relationship between the deterioration rate of heritage materials and the loads of multiple pollutants in combination with various climatic parameters. In Equation (1), the constant 3.95 represents the background value of natural weathering. The study focuses on the limestone materials of heritage structures, which are representative and widely used in the cultural relics of China, often as the constituent materials of foundations or railings of heritage structures. The limestone materials are mainly composed of calcium carbonate (CaCO3) and have similar properties with a density of about 2.65–2.80 g/cm3, Mohs hardness of about 3–4, and porosity as low as 2~4%.
For quantifying the surface corrosion values in different regions, we divided the study area into 79,071 cells with the resolution of approximately 0.1° × 0.1° longitude–latitude grid, corresponding to grid cell sizes of approximately 11 km × 11 km. The surface recession of the material was estimated for each cell for 15 years by applying the above dose–response function. The values of surface recession for each of the 5058 national sites and UNESCO WHL sites were extracted from the layer according to their geographic locations. All the calculations and spatial interpolation were performed using Geographic Information System (GIS) software.
According to the ICP Materials, the corrosion rate of Portland limestone should be controlled to under 8 µm/year for 2020 and 6.4 µm/year for 2050 [63]. In this study, the targets proposed by ICP Materials are considered as the criteria for risk assessment.

2.3. Data

The data on ground-level air pollutants for O3 and PM10 covering the area of China were derived from the China High Air Pollutants (CHAP) dataset for the years 2006–2020 with O3 at a spatial a resolution of 10 km × 10 km and PM10 at a resolution of 1 km × 1 km [64,65]. The climate parameters for annual averages of temperature, relative humidity, and annual precipitation and the average annual concentrations of SO2 and NO2 were derived from the National Earth System Science Data Center, National Science & Technology Infrastructure of China (http://www.geodata.cn (accessed on 1 August 2022)). The dataset covers the entire region of China and includes national seamless ground data from 2006 to 2020, with the climate parameters at a spatial resolution of 1 km × 1 km [66,67,68] and SO2 and NO2 at a spatial resolution of approximately 0.1° × 0.1° [69,70,71,72,73]. The pH values of precipitation were extracted from the Ecological and Environmental Status Bulletin of China (https://www.mee.gov.cn/ (accessed on 1 August 2022)), which contains the distribution of annual average of rainfall pH from 2003 to 2020 [74].
The data of air pollutants and climatic parameters for the year 2006–2020 were first resampled to the same spatial resolution of 0.1° × 0.1° longitude–latitude, corresponding to grid cell sizes of approximately 11 km × 11 km. When NoData occurred in the data source, this was patched with the Focal Statistics tool. The average annual values of corrosion for each cell were returned by applying the Equations (1) and (2) with the raster data on annual averages of the concentrations of air pollutants and meteorological parameters as input data. The grid-by-grid calculations on all the raster layers were performed by the Raster Calculator. All operations were carried out with GIS software.

3. Results

3.1. The Pollutant Emissions in 2006–2020

Figure 2 shows the major pollutant emissions in 2006–2020. China has set binding targets for pollution reduction, including SO2 and NOx, in its Five-Year Plans since 2006, and the pollutant emissions have slowed [75]. In 2013, China launched the battle against air pollution and launched desulfurization, denitrification, and dust removal renovation projects in key industries [57,76,77]. As a result, pollutant emissions have reduced dramatically since 2015.
In terms of different pollutants, SO2 emissions peaked in 2006 due to the accelerated urbanization and rapid economic development [78,79], and decreased to 3.2 million tons in 2020 with a reduction of 87.7% compared to 2006 as a result of desulfurization, denitrification, and dust removal projects in key industries [80]. NOx emissions increased as a result of motor vehicle expansion since 1996 [81] and showed a significant declining trend since 2011 with the widespread implementation of flue gas denitrification and the improvement of motor vehicle emission standards [82]. NOx reduced by 50.9% in 2011–2020. PM10 emissions reached a peak of 18.3 million tons in 2006 with the rapid development of China’s economy and growing energy consumption [78]. The dust removal engineering in key industries, the stricter emission standards, as well as the improvement of energy mix resulted in a 66.2% reduction in PM10 in 2006–2020. The acid rain was controlled with the improvement of air quality [78]. By 2020, the annual average pH value reached 5.6, meeting the threshold for the definition of acid rain.
Sustainability 15 08709 g002 550
Figure 2. The major pollutant emissions in 2006–2020. Note: NOx, SO2 data are from National Bureau of Statistics of China [83]; PM10 data in 2006–2015 are from Multi-resolution Emission Inventory for China (MEIC) [84]; PM10 data in 2016–2020 and average pH data are from the Ecological and Environmental Status Bulletin of China [74].
Figure 2. The major pollutant emissions in 2006–2020. Note: NOx, SO2 data are from National Bureau of Statistics of China [83]; PM10 data in 2006–2015 are from Multi-resolution Emission Inventory for China (MEIC) [84]; PM10 data in 2016–2020 and average pH data are from the Ecological and Environmental Status Bulletin of China [74].
Sustainability 15 08709 g002

3.2. The Recession Depths of Heritage

Figure 3 shows the annual surface recession of limestone material of heritage structures and the difference in 2006–2020. To illustrate the benefits of air quality improvement, the figure divides the temporal scale into three periods: 2006–2010, 2011–2015, and 2015–2020, corresponding to China’s three Five-Year Plans.
In Figure 3a–c, the red area shows the corrosion rate exceeding the ICP Material target for 2020 (8 µm/year), the brown area shows the corrosion rate exceeding the target for 2050 (6.4 µm/year), and the blue area shows the corrosion rate meeting the target for 2050. The average annual corrosion rate of the limestone materials of heritage is estimated to be 9.53 µmy−1 for 2006–2010, 9.14 µmy−1 for 2011–2015, and 7.33 µmy−1 for 2016–2020, showing that corrosion risk is declining. In 15 years, the annual recession rate decreased from 9.69 μmy−1 in 2006 to 6.71 μmy−1 in 2020, an average decrease of 30.1%, confirming the effectiveness of China’s clean air action for heritage conservation.
Figure 3d shows the changes in annual surface recession in 2006–2020. The blue area shows that the corrosion rate decreased and the red area shows that it increased. During this period, 78.6% of the domain showed a decreasing trend in the corrosion level. There has been a great decline in recession rates in the southern regions due to the pollutant reduction, but a significant increase in the Qinghai–Tibet region due to the increase in relative humidity. On the whole, the regions with high recession rates show significant decreases and the regions with low recession rates show rising trends.

3.3. Contribution of Different Pollutants to Heritage Recession

Figure 4 shows the average annual contributions of different pollutants to heritage recession. The yellow bars represent the background weathering, referencing the background value of 3.95 of the first year for weathering proposed by ICP [47]. The result reveals that most recession arises from background weathering, which increased by 9.4% in the past 15 years alongside the reduction in air pollutants. Among all the pollutants, PM10 is the leading contributor to limestone recession, and hydrogen ions contribute the least. In 2006–2020, the reduction in SO2 and PM10 was the major driver of the slowdown of limestone recession. The contribution of nitric acid increased and surpassed SO2 to become the second most important contributor to limestone recession during the period of 2016–2020. The increase in nitric acid is closely related to temperature and motor exhaust, which should be given more attention in the future.
Figure 5 shows the contributions of different pollutants for four major regions. Overall, the corrosion levels in the southern, northwestern, northern, and Qinghai–Tibet regions are descending.
The high corrosion level in the southern region is caused by SO2 and HNO3, which is closely related to the high relative humidity (Figure 5a). The deposition of SO2 and NO2 is influenced by relative humidity since it is the key factor determining the thickness of the moisture layer and the ability to dissolve gases [13]. According to Equation (1), SO2 and HNO3 have fewer corrosive effects on limestone at a relative humidity below 60%. The annual average relative humidity throughout most of the southern region is above 65%, which is a crucial factor in the extremely high corrosion level of SO2 and HNO3 in the region. At the same time, the rainfall pH varies from 4 to 5 in the south and is lower than in the other regions, which also is an important contributor to the high recession level. Although the air pollution in the northern region is heavier than in the southern region, the corrosion levels in the northern region are lower owing to the low relative humidity (Figure 5b).
PM10 is a crucial contributor to surface recession, especially in the northwestern region and the Qinghai–Tibet region (Figure 5c,d). The arid climate with low precipitation, high temperature, and strong winds exacerbates the adverse effects of PM10. Owing to the nationwide clean air action, the corrosion induced by PM10 has shown a significant decreasing trend in all regions since 2015. It is worth mentioning that the effects of SO2 and HNO3 in the Qinghai–Tibet region have increased remarkably since 2015, which is related to the increase in relative humidity caused by complex climate change.

3.4. The Risk Level of Limestone Heritage

Figure 6 shows the percentage of heritage sites in different ranges of surface recession. In 2006–2020, the number of heritage sites with surface recession lower than 6.4 μmy−1 (ICP Materials control targets for 2050) increased by 22.8%, from 5.6% in 2006–2010 to 28.4% in 2016–2020; the number of heritage sites with surface recession lower than 8 μmy−1 (ICP Materials control targets for 2020) increased from 36.6% in 2006–2010 to 78.0% in 2016–2020, an average increase of 41.4%. Although the recession risks of heritage were well controlled in past 15 years, 22% of heritage sites have not met the ICP Material 2020 target and are still at a high risk level.
Table 1 shows the annul recession rates of 35 UNESCO World Heritage List (WHL) sites in three periods, which were extracted according to their geographic locations from the values calculated for each cell hosting the cultural sites. In 2006–2020, most heritage sites showed a decreasing trend in surface recession, except for three sites: the Old Town of Lijiang, Dazu Rock Carvings, and Kulangsu. Owing to air quality improvement, the number of sites with corrosion rates lower than the ICP Material targets for 2020 and 2050 [63] increased by 42.2% and 26.7%, respectively. However, the recession rates of nine sites, including the West Lake Cultural Landscape of Hangzhou and Kaiping Diaolou and Villages, still exceed 8 μmy−1, indicating a high risk level. The recession rates of 21 sites, including Shanhaiguan Great Wall, Ming Xiaoling Tombs, and Fujian Tulou, exceed 6.4 μmy−1 and are at a medium risk level.

4. Discussion

4.1. The Losses Avoided from Air Quality Improvement

In 2006–2010, the annual recession depth of limestone material of heritage structures decreased by 14.55 μm due to the air quality improvement, and the air quality improvement produces huge synergies for heritage conservation. However, it is difficult to estimate the monetary benefit considering the complexity of heritage. This study tried to quantify the benefit by estimating the losses avoided due to air quality improvement.
Theoretically, the losses avoided due to air quality improvement can be expressed as the difference between the actual maintenance costs of heritage conservation and the theoretical maintenance costs. The actual maintenance costs in 2006–2020 are shown by pink bars in Figure 7. The maintenance costs have shown a slow growth trend over 15 years according to the statistics of the National Cultural Heritage Administration. The study employed maintenance intervals to estimate the theoretical maintenance costs without air quality improvement.
The maintenance intervals can be simplified as the quotient of the tolerable degradation depth and the recession rate [63]. The shorter the maintenance interval, the higher the maintenance cost. Theoretically, the maintenance costs without air quality improvement can be estimated as the product of the actual maintenance costs and the expansion factor of the maintenance interval. Assuming that recession rates remain at 2006 levels without pollutant reduction, the expansion factor of the maintenance interval without pollutant reduction relative to the realistic case with pollutant reduction can be obtained at the same level of tolerable degradation depth. The estimated maintenance costs per year in 2006–2020 without air quality improvement are shown in Figure 7. In 2006–2020, the air quality improvement prevented losses of CNY 136.2 million, equivalent to 1.8 years of total maintenance costs for heritage sites. It should be noted that the analysis provides a possible way to quantify the benefit of air quality improvement, but it is uncertain due to the complexity of heritage conservation and the simplification of the method.

4.2. Policy Implications for Heritage Conservation

4.2.1. Paying Attention to the Degradation Risk of Heritage

Risk can be broadly classified as emergency risk and incremental risk based on different latent periods and manifestations [85]. Emergency risk often arises from a shock event and is known as a “black swan” event, while incremental risk displays creeping characteristics and is known as a “gray rhino” event. The degradation risk of heritage structures from air pollution is a typical “gray rhino” event; it is difficult to manage once the degradation depth cross a critical point.
The degradation rate of heritage structures has decreased owing to the air quality improvement over the past 15 years, but the incremental risk remains at a high level. The gradual and cumulative deterioration of heritage structures proceeds with any amount of pollutants [86,87], and the deterioration can be intensified by rising temperatures, prolonged wetness, and the deposition of pollutants [88,89]. Currently, China’s heritage conservation focuses on extreme event response such as collapse induced by heavy rain and takes little consideration of creeping deterioration. The long-lasting and healthy conservation of heritage sites calls for consideration of the degradation risk. In particular, the risk of corrosion and soiling of heritage structures from particulate matter is high in all regions of China, and ongoing prevention and protection based on local risk factors are required.
The risk assessment is the vital first step in addressing the degradation risk. The risk arises from the exposure, the vulnerability of the heritage site itself, as well as the environmental loads [90], providing a helpful framework for the degradation risk assessment of heritage sites [91]. Accurate risk assessment also means that the government should strengthen monitoring and pay special attention to changes in the heritage sites and their environment [92].

4.2.2. Promoting Heritage Conservation through Social Sustainability

Sustainable Development Goal 11.4 aims to strengthen efforts to protect and safeguard the world’s cultural and natural heritage and to achieve targets for social sustainability, including clean air [93]. As built heritage is mostly a simple non-living system that is less capable of responding to adverse external factors such as pollutants, reducing the risk of surface recession depends on human intervention and management. In order to achieve the sustainable development of the heritage sector, heritage management should seek to cooperate not only in pollution reduction, but also energy conservation, climate change mitigation, and other related social goals, considering that heritage conservation has strong synergy with those sustainable development targets [94,95,96].
However, this coordination with other related goals is what is missing in China’s heritage conservation management policy [97]. The government should realize that the achievement of multiple Sustainable Development Goals is consistent with the internal logic and practical path of heritage risk management. This study illustrates the linkage between heritage conservation and air pollution control, but the broader synergies between heritage conservation and other sustainable development goals should also be considered. The heritage site resistance can be enhanced by integrating goals including pollution reduction, carbon emissions reduction, energy saving, economic growth, and sustainable tourism. Only by continuously improving society’s ability to cope with risks can the resilience of heritage sites be truly enhanced [98,99].

4.2.3. Implementing Differentiated Conservation Tactics

The heritage degradation risks show large regional differences (Figure 3), which come from variations in risk factors, including heritage resource density (Figure 1) and regional air pollutant contributions (Figure 5). The regional difference means the government should implement differentiated conservation tactics. The nation should prioritize the allocation of financial resources to regions at high risk, and the local governments should develop flexible solutions according to the key risk factors.
The regions with high pollutant concentrations and degradation levels, such as the southern and northwestern regions, should pay more attention to clean air actions, such as developing environmental standards or setting acceptable pollutant levels for heritage conservation zones, and enhance routine maintenance, including surface protection and cleaning to reduce pollutant soiling. The regions with a high density of heritage resources, such as the central and southeastern regions, should integrate more social resources and stakeholders into the heritage risk governance to enhance heritage resilience. The regions with increased degradation levels, such as the Qinghai–Tibet region, should put more emphasis on balancing environmental protection and economic growth to control the progression of corrosion levels.

4.3. Limitations of the Study

This study attempted to assess the benefits of air quality improvement for heritage conservation. The results provide a perspective for understand the relationship between heritage and air pollution in China, but they should be interpreted with caution due to the limitations of the method. The annual and regional comparisons are more meaningful than specific values.
The dose–response function is involved in the uncertainty of the results. Due to the complexity of the damage mechanism, the function does not take into consideration the natural climatic factors, such as temperature, wind, extremes, and moisture [25,100,101], and the additional damage to limestone in northern regions [102] and additional salt damage at coastal sites [103,104] are also not included. Furthermore, although both Portland limestone and the limestone used in China are composed of calcite (CaCO3), relevant experimental and validation studies on the differences in material composition should be conducted in the future. The employment of yearly mean values of pollutants is also subject to uncertainty given the daily fluctuation in pollutants that have different impacts on heritage degradation. Future research should develop a refined dose–response function to conduct risk assessment, especially for specific heritage sites.

5. Conclusions

Recognizing the dramatic decline in air pollutant emissions and the changing structure of air composition in China over the past decade, this study quantifies the impact of air pollutants on the surface degradation of limestone and assesses the benefits of air quality improvement for heritage conservation. This study highlights the degradation risk of heritage sites and provides evidence of the benefits for heritage conservation from air quality improvement.
In 2006–2020, the widespread implementation of pollution control measures resulted in large reductions of 87.7%, 66.2%, and 22.3% in SO2, PM10, and NOx emissions, respectively. Owing to air quality improvement, the annual recession rate decreased from 9.69 μmy−1 in 2006 to 6.71 μmy−1 in 2020, meeting the ICP Materials control target of 8 µmy−1 for 2020 and nearing the target of 6.4 µmy−1 for 2050. The air quality improvement avoided an average surface degradation depth of 14.55 μm during the 15-year period and has avoided about CNY 136.2 million in heritage site maintenance costs. Although the nationwide risk is decreasing, the risk level shows a significant regional difference. The risk level is descending in southern, northwestern, northern, and Qinghai–Tibet regions. Nationwide, PM10 is the leading contributor to the recession of limestone material of heritage structures in China. For protecting and safeguarding cultural heritage, future policies should pay attention to the degradation risk, promote heritage conservation though social sustainability, and implement differentiated conservation strategies.

Author Contributions

Conceptualization, H.L.; methodology, X.W.; formal analysis, X.W. and Y.W.; resources, X.W. and H.L.; writing—original draft preparation, X.W.; writing—review and editing, Y.W. and X.Z.; supervision, H.L. and X.Z.; funding acquisition, H.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Beijing Municipal Social Science Foundation, grant number 22LSB008.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We also thank the data support from National Earth System Science Data Center, National Science & Technology Infrastructure of China. Available online: http://www.geodata.cn (accessed on 9 November 2022).

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Distribution of the national cultural heritage sites and the UNESCO World Heritage Sites in China.
Figure 1. Distribution of the national cultural heritage sites and the UNESCO World Heritage Sites in China.
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Figure 3. Estimated annual surface recession of limestone material of heritage sites in 2006–2020.
Figure 3. Estimated annual surface recession of limestone material of heritage sites in 2006–2020.
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Figure 4. The contribution of different pollutants to heritage recession. Note: The numbers above the bars represent the average contribution rates of different pollutants.
Figure 4. The contribution of different pollutants to heritage recession. Note: The numbers above the bars represent the average contribution rates of different pollutants.
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Figure 5. The contributions of different pollutants for the 4 major regions in 2006–2010.
Figure 5. The contributions of different pollutants for the 4 major regions in 2006–2010.
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Figure 6. The percentage of heritage sites in different ranges of surface recession in the three 5-year periods.
Figure 6. The percentage of heritage sites in different ranges of surface recession in the three 5-year periods.
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Figure 7. The surface recession rate. The actual maintenance costs and the theoretical maintenance costs in 2006–2020.
Figure 7. The surface recession rate. The actual maintenance costs and the theoretical maintenance costs in 2006–2020.
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Table
Table 1. Corrosion values of cells hosting WHL properties for the three periods.
Table 1. Corrosion values of cells hosting WHL properties for the three periods.
SiteThe Annul Recession Rate of Limestone (μmy−1)
2006–20102011–20152016–2020
The Great WallBadaling Great Wall6.96.75.8
Shanhaiguan Great Wall7.910.06.6
Jiayuguan Great Wall7.17.06.1
Mogao Caves8.48.47.3
Imperial Palaces of the Ming and Qing Dynasties in Beijing and ShenyangImperial Palaces of the Ming and Qing Dynasties in Beijing7.87.56.5
Imperial Palaces of the Ming and Qing Dynasties in Shenyang10.18.46.1
Mausoleum of the First Qin Emperor9.38.77.1
Peking Man Site at Zhoukoudian7.77.46.3
Ancient Building Complex in the Wudang Mountains13.214.17.3
Historic Ensemble of the Potala Palace, Lhasa5.96.05.1
Mountain Resort and its Outlying Temples, Chengde6.66.55.7
Temple and Cemetery of Confucius and the Kong Family Mansion in Qufu12.112.27.4
Lushan National Park9.911.08.7
Ancient City of Ping Yao7.27.26.4
Classical Gardens of Suzhou12.312.17.0
Old Town of Lijiang5.45.16.1
Summer Palace, an Imperial Garden in Beijing7.77.46.9
Temple of Heaven: An Imperial Sacrificial Altar in Beijing7.87.56.4
Dazu Rock Carvings7.88.08.9
Ancient Villages in Southern Anhui—Xidi and Hongcun10.09.28.2
Longmen Grottoes8.48.36.9
Imperial Tombs of the Ming and Qing DynastiesEastern and Western Qing Tombs7.77.56.4
Ming Xiaoling Tombs11.611.47.8
Ming Tombs7.17.06.1
Three Tombs of Shengjing9.48.06.2
Xianling Tombs11.511.07.9
Mount Qingcheng and the Dujiangyan Irrigation System9.79.17.4
Yungang Grottoes6.76.56.0
Capital Cities and Tombs of the Ancient Koguryo KingdomJilin Province section8.57.97.1
Liaoning Province section8.97.86.7
Yin Xu10.48.77.2
Fujian Tulou8.49.26.9
Kaiping Diaolou and Villages10.511.89.0
Mount Wutai6.66.76.0
Historic Monuments of Dengfeng in “The Centre of Heaven and Earth”8.78.46.6
Cultural Landscape of Honghe Hani Rice Terraces9.69.57.6
West Lake Cultural Landscape of Hangzhou12.012.19.4
Site of Xanadu6.36.25.5
Tusi SitesSites of Laosicheng11.09.47.2
Sites of Hailongtun11.410.66.9
Sites of Tangya10.48.87.1
Zuojiang Huashan Rock Art Cultural Landscape8.79.98.9
Kulangsu, a Historic International Settlement7.97.98.1
Archaeological Ruins of Liangzhu City12.512.38.2
Quanzhou: Emporium of the World in Song-Yuan China9.311.66.0
Note: Orange cells refer to corrosion rates greater than the ICP Material 2020 target; green cells refer to corrosion rates lower than the ICP Material 2050 target; yellow cells refer to corrosion rates between the ICP Material 2050 and 2020 targets.
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Wang, X.; Li, H.; Wang, Y.; Zhao, X. Quantifying the Potential Co-Benefit of Air Quality Improvement on Cultural Heritage in China. Sustainability 2023, 15, 8709. https://doi.org/10.3390/su15118709

AMA Style

Wang X, Li H, Wang Y, Zhao X. Quantifying the Potential Co-Benefit of Air Quality Improvement on Cultural Heritage in China. Sustainability. 2023; 15(11):8709. https://doi.org/10.3390/su15118709

Chicago/Turabian Style

Wang, Ximo, Huimin Li, Yufei Wang, and Xiaofan Zhao. 2023. "Quantifying the Potential Co-Benefit of Air Quality Improvement on Cultural Heritage in China" Sustainability 15, no. 11: 8709. https://doi.org/10.3390/su15118709

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